Photovoltaics is concerned with direct conversion of light or solar energy into electricity through the use of active electronic devices called solar cells. Solar cells are commonly fabricated on wafers of polycrystalline or single crystal silicon. However, the cost of electricity generated using silicon-based solar cells is rather high, compared to electricity available from typical electrical power system grids. The production cost of the photovoltaic product is often represented in units of dollars per watt generated under standard 100 mW/cm.sup.2 illumination intensity.
To make thin film solar cell technology competitive with common silicon-based photovoltaic products and with the traditional methods of electric power generation, a film growth technique has to be developed that can deposit solar cell grade, electronically active layers of the absorber materials and other components of the solar cells on large area substrates, using cost effective approaches with high throughput and high materials utilization. Therefore, there has been a continuous effort to develop low cost solar cells, based on thin film polycrystalline compound semiconductor absorber layers.
Group IB-IIIA-VIA materials are promising as the absorber layers of high efficiency thin film solar cells. In fact, a comparatively high efficiency thin film device has already been produced on a Cu(In,Ga)Se.sub.2 absorber film grown by a vacuum evaporation technique. A demonstrated conversion efficiency of over 17% confirmed the capability of this material to yield quite efficient active devices when employed in thin film solar cell structures.
The electrical and optical properties of Group IB-IIIA-VIA compound films depend on their chemical composition, defect chemistry and structure, which in turn are related to the film growth techniques and parameters. There are a variety of deposition techniques that have been used for the growth of Group IB-IIIA-VIA compound semiconductor films. However, it is crucial to obtain a material that has the good opto-electronic and structural properties which are needed for the production of active electronic devices such as solar cells.
In solar cells based on a Group IB-IIIA-VIA absorber film, appreciable amounts of the binary phases, such as Group IIIA-VIA compounds and especially Group IB-VIA compounds, in the absorber film, typically deteriorate the electronic properties of the compound, and thus the characteristics of the solar cells. In addition, it is considered desirable to have an absorber material with columnar grains equivalent to at least about 0.5 .mu.m diameter, in thin film solar cell structures. Furthermore, for commercial viability, the technique should be able to deposit a layer that is relatively uniform compositionally onto very large substrates, such as several ft.sup.2 in area, using low cost equipment and processes.
A significant compositional parameter of Group IB-IIIA-VIA thin films is the molar ratio of the Group IB element or elements to the Group IIIA element or elements. This is commonly referred to as the I/III ratio. Typically an acceptable range of the I/III molar ratio for the Cu-containing solar cell using Group IB-IIIA-VIA materials is about 0.80-1.0, although in some cases involving extrinsic doping with a dopant such as Na, this ratio can go even lower to about 0.6. If the I/III ratio exceeds 1.0, a low resistivity copper selenide phase typically precipitates and deteriorates the performance of the device.
One technique that has yielded relatively good quality Group IB-IIIA-VIA films for solar cell fabrication is co-evaporation of Group IB, IIIA and VIA elements onto heated substrates. As described by Bloss et al. in their review article ("Thin Film Solar Cells", Progress in Photovoltaics, vol. 3, pages 3-24, 1995), the film growth in this technique takes place in a high vacuum chamber and the evaporation rates of the Group IB and Group IIIA elements are carefully controlled to keep the overall I/III ratio of the film in the acceptable range.
However, the evaporation method is not readily adaptable to low cost production of large area films, mainly because uniform deposition by evaporation on large area substrates is difficult, and the cost of vacuum equipment is high. Co-sputtering of Group IB and Group IIIA elements such as Cu and in the presence of Group VIA vapors such as Se, has also been investigated as a possible method of compound film growth. However, this technique suffers from yield problems, most probably due to poor capability to control the I/III ratio.
Another technique for growing Group IB-IIIA-VIA compound thin films for solar cells is a two-stage process where at least two components of the Group IB-IIIA-VIA material are first deposited onto a substrate, and then reacted with each other and/or with a reactive atmosphere in a high temperature annealing process. U.S. Pat. No. 4,581,108 issued to Vijay K. Kapur et al. in 1986, U.S. Pat. No. 4,798,660 issued to James H. Ermer et al. in 1989, and U.S. Pat. No. 5,028,274 issued to Bulent M. Basol et al. in 1991 teach respectively the methods of electrodeposition of Group IB and IIIA elements onto a substrate followed by selenization or sulfidation, DC magnetron sputtering of Cu and In layers on a substrate followed by selenization, and deposition of Group IB and IIIA elements onto a substrate previously coated with a thin Te film followed by selenization or sulfidation.
In the two-stage processes, large area magnetron sputtering techniques can be used to deposit individual layers containing Group IB and Group IIIA elements for precursor film preparation. In the case of CuInSe.sub.2 growth, for example, Cu and In layers can be sputter-deposited on non-heated substrates and then the composite film can be selenized in H.sub.2 Se gas or Se vapor at an elevated temperature, as is shown in U.S. Pat. Nos. 4,798,660 and 5,028,274.
The film growth techniques require strict control of the material composition during the deposition process, with a typical goal that in the final film, the overall I/III ratio be in the acceptable range of about 0.80-1.0. For mass production of photovoltaic modules, this ratio should be uniform over large area substrates. In the two- stage processes the uniformity and thickness of each layer has to be controlled.
When the I/III ratio is greater than 1.0, it causes the separation of a Cu-selenide phase in Group IB-IIIA-VIA compound layers. Layers containing Cu-selenide phase have low resistivities and typically are not used in active device fabrication. However, these Cu-rich films have good structural characteristics and large grain sizes. The relationship between the structural properties of Group IB-IIIA-VIA materials and their composition can be used beneficially, especially in the co-evaporation approaches, by intentionally increasing the I/III ratio above 1.0 during the film growth process for improving the structural properties of the growing film, and then decreasing it back to the acceptable range by the time the deposition process is terminated. Films grown by such approaches often have large grain sizes and good electronic properties. Therefore, it is typically allowable to change the I/III ratio during the deposition and growth of a Group IB-IIIA-VIA compound, but with the overall ratio in the final film being within the 0.80-1.0 range.
Since the uniformity and control of the I/III ratio throughout the material is important for Group IB-IIIA-VIA compounds, attempts have been made to fix this ratio in a material, before the deposition process, and then transfer this fixed composition into the thin film formed using the material. Early attempts for CuInSe.sub.2 growth by such an approach were by evaporation and by sputtering, using pre-formed CuInSe.sub.2 compound material as the evaporation source or the sputtering target. However, these efforts did not yield solar cell grade material because of the lack of reproducible compositional control, which may have resulted from Se and/or In.sub.2 Se loss in the vacuum environment. In the case of sputtering, the changing nature of the target surface also presented a problem. A relatively efficient solar cell was recently demonstrated on a layer obtained by laser ablation of a CuInSe.sub.2 target (H. Dittrich et al., 23rd IEEE PV Specialists Conference, 1993, page 617), however, such an approach is not practical for large scale production.
Other attempts to prepare Group IB-IIIA-VIA compound films using a material with a pre-fixed composition have included screen printing layers onto substrates and their conversion into the compound. T. Arita et al. in their 1988 publication (20th IEEE PV Specialists Conference, 1988, page 1650) described a screen printing technique that involved: creating an initial material by mixing pure Cu, In and Se powders in the compositional ratio of 1:1:2, milling these powders in a ball mill and forming a screen printable paste, screen printing the paste on a substrate, and sintering this precursor film to form the compound layer. The milling was done in a media such as water or ethylene glycol monophenyl ether to reduce the particle size, and formation of a paste was done using a propylene glycol binder. The paste material was deposited on a high temperature borosilicate glass substrate by the screen printing method, forming a film. The post-deposition treatment step consisted of annealing the film in nitrogen gas at 700.degree. C., to form a compound film on the substrate.
For evaluating the photovoltaic characteristics of the resulting compound, thick pellets were made from the material obtained as a result of the milling and sintering steps, and solar cells were fabricated on them. Efficiencies of only about 1% were reported for these devices. The researchers further reported that CdS/CuInSe.sub.2 thin film junctions were also fabricated by depositing CdS film on the sintered CuInSe.sub.2 films by RF sputtering, but concluded that they were not able to obtain better photovoltaic characteristics than that in pellet samples. Their reported data indicated that In powder was oxidized during the milling process, that Cu, In and Se were reacting with each other during milling, and that the CuInSe.sub.2 material obtained after the sintering process had a resistivity of about 1.0 ohm-cm. This resistivity is only about 0.01-1% of the value for a typical CuInSe.sub.2 film that yields efficient solar cells and may indicate the presence of a detrimental Cu--Se phase. Also, the sintering temperature of 700.degree. C. is very high for low cost solar cell structures that employ soda-lime glass substrates.
Thin layers of CuInSe.sub.2 deposited by a screen printing method were also reported by a research group at Universiteit Gent in Belgium. A. Vervaet et al., in their 1989 publication (9th European Communities PV Solar Energy Conference, 1989, page 480), referring to the work of T. Arita et al., indicated that indium powder easily oxidizes, giving rise to unwanted phases, such as In(OH).sub.3 or In.sub.2 O .sub.3 in the final films. The technique of the Universiteit Gent research group, therefore, employed the steps of: forming a CuInSe.sub.2 powder as an initial material by crushing a CuInSe.sub.2 ingot; grinding the CuInSe.sub.2 powder in a ball mill; adding excess Se powder and other agents such as 1,2-propanediol into the formulation to prepare a screen printable paste; screen printing layers onto borosilicate and alumina substrates; and high temperature sintering of the layers (above 500.degree. C.) to form the compound films. A difficulty in this approach was finding a suitable sintering aid or fluxing agent for CuInSe.sub.2 film formation. Among many agents studied, copper selenide was the best for grain growth, but films containing this phase could not be used for active device fabrication since they had I/III ratios larger than 1.0.
More recently, the Universiteit Gent group experimented with CuTlSe.sub.2, a compound with a relatively low (about 400.degree. C.) melting point, as a fluxing agent. In their 1994 publication (12th European PV Solar Energy Conference, 1994, page 604), M. Casteleyn et al., used CuTlSe.sub.2 in their formulation of the CuInSe.sub.2 paste, and demonstrated grain growth for films with I/III ratios in an acceptable range. However, the solar cells fabricated on the resulting layers were still poor with conversion efficiencies of only of about 1%. The sintering temperature of above 600.degree. C. used in this process was also high for low cost glass substrates. Using CuInSe.sub.2 powder as the initial material did not produce good results because of the lack of a good sintering aid that would not deleteriously affect the electronic properties of the final film obtained by this technique. The sintering temperature employed in the above referenced screen printing techniques was very high (&gt;600.degree. C.) for the use of low cost substrates for the deposition of Group IB-IIIA-VIA compound films.
In addition to difficulties associated with controlling the macro-scale uniformity of the I/III ratio over large area substrates, there are also concerns involving micro-scale non-uniformities in Group IB-IIIA-VIA compound thin films. In U.S. Pat. No. 5,445,847, issued to T. Wada et al. in 1995, the researchers treated a Group IB element layer and a Group IIIA element layer with heat under the presence of the chalcogen to obtain a chalcopyrite-type compound. They observed a deviation in a composition ratio of the Group IB element to the Group IIIA element in the obtained compound, and stated that the composition itself was not always microscopically constant. As a remedy to this problem they used a Group IB-IIIA oxide composition, which has a high melting temperature, instead of the element layers. They concluded that the Group IB-IIIA oxide composition did not melt from the heat treatment temperature under a reducing atmosphere containing the Group VIA element or containing the reducing compound of the Group VIA element, and that the initial composition can be maintained in micro-scale. X-ray diffraction data indicated the formation of the Group IB-IIIA-VIA phase. However, apparently no data has been published on the electronic qualities of these layers, and no active devices such as solar cells have been fabricated.
Another approach concerning the micro-scale control of the I/III ratio is indicated in European Patent No. 93116575.7 (Publication No. 0595115A1, 1994) of T. Wada et al. There, a chalcopyrite-type compound is prepared by annealing a thin film containing Cu, In and an In compound or a compound which contains both In and Cu, selected from the group consisting of oxides, sulfides and selenides, in an atmosphere containing a Group VIA element. It is concluded that since a Cu/In ratio of less than about 1.0 is desired for a solar cell grade CuInSe.sub.2 compound film, excess Group IIIA element In had to be present prior to annealing. According to these researchers In would give rise to microscopic non-uniformities in the layers because of its low melting point. Therefore, the idea was to replace In with its high melting point oxide, sulfide or selenide. This was achieved by depositing multi-layers onto substrates, just as in two-stage processes, and by their reaction to form the desired compounds. Some of the examples of multi-layer depositions include electron-beam evaporation or sputtering of a Cu layer and an In layer, followed by sputtering or laser ablation of an indium oxide, indium sulfide or indium selenide, co-deposition of a Cu.sub.11 In.sub.9 alloy layer followed by the deposition of a film of an oxide, selenide or sulfide of In, deposition of a Cu layer and an In layer, followed by the deposition of an indium oxide and then a copper oxide layer, or an indium selenide and a copper selenide layer, or an indium sulfide and a copper indium sulfide layer.
A processing technique employing multi-layers of deposited materials containing the Group IB and IIIA elements may address the issue of micro-scale compositional uniformity by including high melting point compounds in the layers, however, just as in simpler two-stage processes, would not be expected to address the more important issue of macro-scale uniformity of the I/III ratio. In other words, if multi layers containing Group IB and IIIA elements need to be deposited on a large area substrate, the thickness and the thickness uniformity of each layer containing the Group IB and/or IIIA elements requires strict control. In that respect, compositional control for, e.g., Cu/In/In.sub.2 O.sub.3 or a Cu/In/Cu.sub.2 O.sub.5 stack, for example, is more complicated than for the Cu/In stack of the simple two-stage process.
As the above review of prior art demonstrates, there is a need for techniques to provide Group IB-IIIA-VIA (and related) compound films on large area substrates, with good compositional uniformity. There is also a need for such compound films with superior electronic properties, that would make them suitable for the fabrication of active electronic devices such as solar cells.